Recently, communication services of which a communication capacity is very large, including a video of a smart phone, or the like, have been released. Therefore, the necessity to increase an existing communication capacity is emerging, and a dense wavelength division multiplexing (DWDM) communication method has been adopted as a method of increasing the communication capacity using an optical fiber buried in advance. The DWDM communication method indicates a method of simultaneously transmitting beams having several wavelengths by one optical fiber using a phenomenon that laser beams having different wavelengths do not interfere with each other, such that interference between beam signals is not generated even though the beam signals having several wavelengths are simultaneously transmitted through one optical fiber. In the DWDM communication method recently used in optical communication, an interval of 100 GHz or 50 GHz has been used as a frequency interval of a beam, and this frequency interval has a wavelength interval of about 0.8 nm, or 0.4 nm in a wavelength band of 1.55 μm.
In the DWDM communication method, communication wavelengths of at least 32 to 40 channels are present in a C-band (1520 to 1560 nm), and 20 more kinds of DFB-LD chip which is applied communication wavelengths of at least 32 to 40 channels sets are required
The DFB-LDs are subjected to a complicated semiconductor manufacturing process, and each of the DFB-LDs has different specific wavelengths. Therefore, it was difficult to implement chip sets corresponding to all DWDM channels using the DFB-LD chips. In order to solve this problem, the present inventor has suggested a transistor outline (TO) type external cavity laser structure of which a wavelength is determined at the outside in Korean Patent Publication No. 10-1124173. The external cavity type laser, which is a method of determining an oscillation wavelength of the semiconductor laser at the outside of a semiconductor laser diode chip, may allow the semiconductor laser to have several wavelengths depending on a method of determining the wavelength at the outside, or allow oscillation to be generated at only a specific wavelength. Generally, the external cavity type laser has been mounted and implemented in a package housing called a butterfly package. The butterfly package has a very large volume, and a cost of the package housing itself is very expensive.
In a description of the present invention, in the external cavity type laser, a semiconductor diode chip acts as only a gain chip providing a gain before a laser operation occurs, and may be called a laser diode chip in the case in which the gain chip is wavelength-locked to a wavelength selected at the outside to perform the laser operation. However, although amplification is generated at the wavelength selected at the outside, such that the wavelength is locked, there is no change in an external structure acting as a gain medium before the wavelength is locked. Therefore, in the present invention, concepts of the gain chip and the laser diode chip will be used together.
As a method of implementing the external cavity type laser in a TO type package, Korean Patent Publication No. 10-1121173 invented by the present inventor illustrates a method of manufacturing the external cavity type laser using the TO type package. FIG. 1 illustrates a structural view of Korean Patent Publication No. 10-1124173 invented by the present inventor. However, in this method, an oscillation wavelength is finely changed depending on an internal temperature or a magnitude of a current flowing to the laser diode chip.
FIG. 2 illustrates characteristics according to the related art that the oscillation wavelength is changed depending on a change in a temperature of a thermoelectric element mounted in the TO type package in the structure of FIG. 1 as described in Korean Patent Publication No. 10-1124173. It may be appreciated in FIG. 2 that a laser oscillation wavelength is not stable, but is vibrated in a wavelength range of 100 pm, depending on a change in an environment in the external cavity type TO type laser structure.
Currently, in the DWDM, a wavelength interval tends to be gradually narrowed to 100 GHz, 50 GHz, and 25 GHz, and a more precise wavelength control is required so that wavelength stability is +/−100 pm, +/−50 pm, and +/−25 pm for the purpose of the DWDM of this wavelength interval.
The TO type external cavity type laser according to the related art including Korean Patent Publication No. 10-1124173 does not have any wavemeter, such that it may not stabilize the wavelength of a laser beam emitted from the TO type laser. Therefore, it is difficult to apply the TO type external cavity type laser according to the related art to an application in which more precise wavelength control is required.